Process of preparing wood pulp



Patented Mar. 23, 1943 UNITED STATES -PATENT OFFICE PROCESS or PREPARINGwoon PULP Fredrick Olsen, East Alton, Ill., assignor to The CelluloseResearch Corporation, East Alton, 111., a corporation of Delaware NoDrawing. Application January 20, 1939,

' Serial No.-252,042

2 Claims. (01. 92-16) I This invention relates to the preparation ofhigh grade cellulosic material from wood.

The object of this invention is to provide a process-for the productionof wood pulp having a high alpha cellulose content.

Another object of this invention is to provide a process for theproduction of wood pulp of high quality, suitable for use in preparingcellulose derivatives; for example, cellulose nitrate or xanthate.Another object of this invention is theproduction of high quality woodpulp for use in the manufacture of paper having good stability.

The process comprises gradually reducing the soda soluble and pentosancontent of the wood pulp and concomitantly attaining a high alphacellulose content, usually above 98%. The wood cellulose produced is ahomogeneous product which renders it well adapted for derivativepurposes and for the preparation of high stability p p p p- The processof this invention subjects the wood tain a condition of maximum puritywithout the formation of undesirable degradation products. This is indirect contrast with prior and elevated temperature and thereaftersubjecting v the bleached pulp to a mercerizing treatment;

When especially high whiteness and low pentosan content of pulp isdesired, the mercerized pulp after water washing is. thereafter againmercerized.

Forthe purpose of removing certain impurireactive strength, and at thesame time nearly to a series of treatments specially designed toatpresent day practice wherein the entire burden v of purifying thecellulose from the very complex, woody structure rests on only a fewprocessing steps, necessarily drastic in character, in order to obtainhigh purity; or where mild in character, resulting in a sacrifice ofpurity of product. The conventional manufacturing technique in thepreparation of wood pulp is based upon the use of a very small number ofpieces of standard equipment; viz., digesters, washers and bleachers.The engineering plan back of such equipment is evidently the productionof as large yields as possible of white fiber. Even the cotton lintersThis series of treatments comprises impregnating wood with an aqueoussolution, such as an acid or alkaline delignifying reagent, at atemperature lower than that at which lignin is removed from the wood,digesting the impregnated wood at suitably elevated temperature until asumcient amount of the inter-fiber binding material has been removed topermit separation of the wood into individual fibers, bleaching the woodpulp so obtainedby treatments including chlorination, alkali andhypochlorite, treating the bleached pulp with-caustic soda solution atuniform distribution of the delignifying chemicals throughout allportions of the wood, is attained before providing reactivetemperatures, so that when the proper temperature is reached, thereaction proceeds rapidly and simultaneously throughout all portions ofthe wood, including the difiicultly penetrable centers 'of the woodsubdivision.

Before such uniform impregnation can be attained, it has been foundnecessary to remove the air from the interstices of the wood. Thisisaccomplished by first boiling the wood suitably purified for use inderivatives employ substansubdivided in a liquid wherein the vapor ofthe liquid replaces the air within the interstices of the wood. Theliquid maybe the delignifying reagent itself, such as a. caustic sodasolution, kraft liquor and the like, orit may be water; or it may be anelectrolyte solution having a con- .centration of electrolyte tending toinhibit any reaction or adsorption of the delignifying reagent atboiling temperature.

After the removal of air from the subdivisic as, they are maintainedsubmerged in the delignifying reagent of suitable concentration to givethe desired bleachability in the cooking schedule employed until theyare uniformly impregnated raised and the digestion continued until thedesired bleachability of raw material is attained.

The viscosity of cellulose solutions has long been considered asassociated with a state of complexity of the cellulose molecule. The

.- change in viscosity of the cellulose molecule .seems 'tolrequire twoconceptions:

After the wood has been uniformly valence forces. This action will bereferred to as "splitting."

2. The reduction in viscosity obtained by depolymerization, whichinvolves breaking the primary valence bonds and'reducing thereby thechain lengths of the cellulose molecules. This action is described asscission.

The purification treatments of the wood pulp obtained as described aboveare preferably car'- ried out under conditions which result in alteringthe viscosity by splitting of the secondary valence forces rather thanby scission of the primary rinated lignin or the like, are then removedfrom the pulp, for instance, employing a dilute alkaline solution andwater washing.

The bleaching treatments given the pulp are preferably very mild intheir nature; the conditions being controlled to avoid any degradationother than that necessitated by the reduction in viscosity desired.

The treatment of the bleached pulp with alkali at elevated temperatureis carried out in such a manner as to remove the maximum amount ofnon-cellulosic impurities with minimum degradation of the cellulose. Ithas been found that concentrations of 5%-9% sodium hydroxide in solutionare preferable.

When mercerizing the pulp, concentrations of caustic soda from %-18% andtemperatures in the range of 20-40 C. have been found satisfactory.

When wood is subjected to the successive treatments described above, theresulting cellulose product is in an ideal condition of purity,molecular complexity and permeability to reagents for' conversion intohigh quality derivatives. The physical state of the fiber is such thatreagents penetrate readily into the individual cellpartialkaline andacid delignifying reagents, but in any case the practiceof properlyrelating the reagents and woody material prior to attaining reactivetemperatures by the removal and exclusion ofthe normal air content ofthe wood is important where uniformity in impregnation is v to beattained.

When the pulping of the wood and its purifica-- tion have been conductedunder especially lenient conditions, as described above, designed toeffect a separation of the bundles of long chain molecules by splitting,i. e., destroying the secondary valence forces and largely avoiding thedepolymerization of the cellulose through scission'or the breaking ofprimary-valence bonds, the effect of such uniform, mild treatment on thepulp is to attain an end product distinguished from all products of theprior art processes, among -ther things, by its homogeneity.

The following examples describe two specific embodiments of theinvention, but it is to be understood that the invention is not limit dto the specific details set forth therein. I

I may use maple, aspen, tulip and other varieties of poplar, yellowbirch and other suitable Example A Yellow birch, for example, is reducedeither to the flake or chip form of subdivision. The digester is filledwith yellow birch chips of approximately half inch length, and the chipsare and a caustic soda solution added until the ratio of liquor to woodis 3.25 pounds to 1 pound. The concentration of the caustic solution isso adiusted that after taking into consideration the amount ofliquidpresent in the pretreated chips, the ratio of chemical to wood in thesystem will be approximately 0.2 pound of NaOH to 1 pound of wood. Ifthe greater degree of homogeneity of product resulting from digestionwhich has been performed on chips preimpregnated with an electrolyte asdescribed above is not required, this .electrolyte treatment may beomitted. In this case, the chips are boiled in the alkaline digestingliquor of approximately 0.2 pound NaOH to 1 pound of wood (ratio ofliquor to wood approximately 3.25 to 1) under freely vented conditions,or as described in copending application, Serial 17,894, filed April.24, 1935, now issued as U. S. 2,137,779, until uniform distribution ofthe delignifying agent throughout the wood has been secured.

- The caustic soda solution is added to the digester at 70 C. or higher,and after the chips are submerged by this liquor, heating andcirculation are started by means of a circulation pump which draws offthe liquor at the bottom of the digester and charges it into the topthrough a circular perforated pipeline located at the top of thecylindrical section of the digester while high pres-' sure steam issimultaneously introduced into the bottom of the digester at atemperature of approximately 200 C. The relief valve on the digester is"cracked" throughout the initial steaming to permit the air above theliquor level to escape and avoid any false pressure. The temperaturerise of the mass is halted at about 96-98 C. and maintained at thispoint for one hour. This pause permits the caustic to penetrate thechips without reaction and at a temperature below boiling, tending topreserve the electrolyte deposit, and at the same time sufficiently highto expedite the entry of the caustic into the interstices of the wood.The relief valve is then closed and the temperature raised to C. asrapidly as the facilities will permit, the usual requirement being from1 to 2 hours. The temperature is held between the limits of 160 C. i 2"for 6 hours. The liquor circulation pump is stopped 15-20 minutes beforethe termination of the cook so that incoming steam maybreak up anypacking of the pulp that sometimes/occurs and tends to prevent theblowing of the digester cleanly. At

the end of the cooking time, steam is shut ofi and the pressure in thedigester, which is usually about 75 pounds per square inch, is utilizedto discharge the pulp through the blow valve located in the bottom ofthe digester, the stream of pulp usually being directed at a steeltarget in the,

blowpit, which impact serves to largely break down the chips into a,pulpy mass. As soon as the blow valve is opened, waste black liquorfrom a previous digestion is pumped into the digester for about l5minutes to assist in attaining a clean blow or discharge of the pulp.The pulp digestion liquors and particularly when electrolytepreimpregnation has been practiced has effected the production of a rawpulp which is more homogeneous in composition and structure than thatresulting from conventional digestion practices. The impurities whichremain are more uniformly distributed throughout the pulp and morereadily accessible to the subsequent purification reagents. Unless-thedigestion treatment has produced a raw pulp of this type, subsequentpurification steps are inadequate in efiecting proper removal of certainkinds of impurities.

' The pulp is now ready for purification and is is-then added'to thepulp slurry; the consistency having been adjusted to 3.5% and thetemperature to 21-23 C. and then allowed to react for approximately 4hours. The bleaching powder is addedin the form of a solution containing58-62 grams of bleaching powder per liter and saturated with calciumhydroxide. I have found that the degrading effect of the oxidant can beheld at'a minimum by bleaching at relatively low temperatures, forexample, -25 C. and at high pH, for instance 8.5-l1.0 units. After thecompletion of the bleach, the pulp is washed free of available chlorine,as indicated by testing with starch iodide paper.

After this washing treatment, the pulp is im mersed in a concentratedsodium hydroxide solution of 7.0%:L-1% at a consistency of 4% for aperiod of 3 hours at 100 C. This treatment should be carried out underconditions" which will allow the minimum amount of atmospheric oxygen tocome in contact with the pulp. When the alkaline boil is completed, thesolution is drawn off and the pulp washed with purified water until thepH has been reduced to' between 7.0

and 8.0 units. This sodium hydroxide treatment effects important changesinthe alpha and soda soluble contents of the pulp.

, desired, additional purification treatments are first treated at =3.5%consistency with the equivalent of an amount of standard bleachingpowder in the form of chlorine water which will substantially exhaust in20-30 minutes. The treatment is preferably carried out at 20-22 C. Inorder to utilize the acid formed by the chlorination treatments, as ameans of removing ash from the cellulose, the treatment is allowed tocontinue for a total of 45 minutes. A convenient and desirableconcentration of chlorine water to use is 5 grams of chlorine per liter.

At the end of the 45 minute chlorination treatment time, a sufficientamount of sodium hydroxide is added 'to the slurry to give a 1% NaOHconcentration and a treatment consistency of 2%.

- It is desirable to keep the concentration of the sodium hydroxidesolution added to the pulp color, and this-results in a reduction in theviscosity of the final product. The pulp is treated for minutes at 100C. The mass of pulp is then promptly transferred to a continuousfiltering device or a centrifuge and washed thoroughly with purifiedwater until the pH of the slurry is between 1.0-8.0 units.

The pH of the pulp slurry coming from th diluted alkaline boil i averyimportant factor essential, and the treatment with calcium hypochloritepreviously described is now repeated.-

After the second bleaching treatment, the pulp is thoroughly washed inpurified water as after the first bleaching. It is then immensed andthoroughly agitated in a 13.7% NaOH solution at 25 C. for 1 hour at 4%consistency, after which the caustic soda is removed by draining in asettling tank or any suitable apparatus. After the draining has beenaccomplished, the caustic soda. retained by the pulp isuniformly washedout with purified water until the pH is between 7.0 and 10.0 units. Itis important that any treatment of the cellulose in strong NaOHsolutions be carried out uniformly with respect to each fiber sothat theeffect of the treatment is fully reflected in the condition of theindividual fibers rather than in the average condition of the mass ofcellulose as a whole, The uniform physical state of theindividual-fibers imparted by the mercerizing treatment is an especlallyfavorable condition for subsequent reand must be.close ly controlled. Ifthe pH varies a value in-therange desired in the final product,

action; for example, nitration, and it is necessary to carefullymaintain this condition throughout the remaining processing steps of thepurification. v

ficient apparatus for eflecting the desired separation consists of arubber lined tank equipped with agitation of Allegheny metalconstruction,

and an Allegheny metal mesh screen over the top of the vessel. The pulp,as a l%-.- 2%"sli 1rry in purified water, is fed continuously into thetank, the level being maintained at such a height that the agitationthrows the slurry against the screen covering and imparts thereto apulsating action. The continuous agitation prevents the formation of amat of pulp on the screen; the pulsating action imparted to the slurrydischarges the fines through the'screen, and the water in the slurrywashes the fines from the upper surface of the screen, which isinstalled at a slight angle for this purpose. The long-fibered fractionof the pulp is discharged intoan appropriate vessel from theclassification tank and then to further reduce the ash content, istreated with percent (0.25%) concentration of hydrochloric or aceticacid at a 3% consistency for minutes at 23 .C., after which the pulp iswashed with purified wa- -ter which is of a pH of 7.2-7.6, low in totalsolids,

iron, copper and other metallic impurities, until the acid concentrationhas been reduced to approximately 0.05% and then dewatered.

Sufiicient hot purified water at 60-65 C. and

at a pH of 7.0-8.0 is then added to give a 3% slurry and the massagitated for minutes, dewatered by centrifuging, and then cooled toapproximately 20 C. with cold purified water and dewatered again toabout 50% moisture content. The water wet pulp will have a pH between7.0 and 8.0 units.

All of the preceding treatments are preferably carried out under preciseconditions of control with respect to:

1. Concentration of the solutions 2. Quality of the reagents, includingwater 3. Consistency of the pulp slurries 4. pH of treating liquors andwash waters 5. Time and temperature of treatments 6. Cleanliness of airthroughout buildings and equipment 7. Minimum exposure of pulpto air,especially under alkaline conditions and/or higher than room temperature8. Maintenance of pulp moisture above approximately 50%. p

The alpha cellulose content of the resulting product is greater than 98%and soda solubility is less than 4%.

concentration gives the greatest latitude in obtaining the desiredviscosity.

In the foregoing example, the consistency of the pulp in the varioustreatments may vary quite considerably, depending upon the type ofequipment in which the treatments are carried out; the only requisitebeing that the pulp receive uniform treatment with the reagent employed.In the 1% boiling treatment following chlorination, the time required tobring the temperature of the mass of pulp and alkaline liquor to 100 C.is preferably held between 30 and 120 minutes. In the mercerizingtreatment, the time of mercerization is not critical; for instance, itmay vary from /2-4 hours. Example B In this example is described thepurification of unbleached kraft pulp having the following analysis:

Alpha approximat per cent 92 Soda solubility do 9 Viscosity "approximatecentipoises 32 Ash approximate per cent *0.8 Bleachability do 36Pentosan do 11 In the specific example being quoted, the ash coniainediron and copper in amounts oi 48 P. P. M. and 10 P. I. M., respectively,based on the dry weight of cellulose.

The pulp, after complete defibering, was chlorinated by adding an amountof chlorine in the form of chlorine water equivalent to 16% of thebleach value of the pulp, and the mass allowed to stand for thetotaltime of minutes. At the end of this period of time, the pulp was washedthoroughly with water and sufiicient sodium hydroxide added to the pulp.slurry to form a 1% caustic solution. The pulp was then boiled in thiscaustic solution for /2 hour. The pulp was then washed neutral withwater and treated at 2.7% consistency with 6% calcium hypochlorite basedon the Weight of oven dry pulp for 4 hours. The

' pulp was then dewatered, washed with water and The desirable point inthe purification treat-- ment for controlling the cuprammonium viscosityof the cellulose is during the calcium hypochlorite treatment precedingthe treatment with 7% NaOH at 100 C. It is understood that varia: tionsin the wood and in the methods of digestion may result in pulps varyingquite widely in their viscosity values immediately subsequent todigest-10H d pr or to the first chlorine water treatment. Nevertheless,the nature of any particular lot of wood is determined and the digestionadjustedjo give crude pulps having substantionally uniform viscosityvalues. The chlorination and the subsequent causticization are alsocarefully controlled so that the viscosity values are not less thanthose of the raw pulp immedir at-ely after the digestion.

The prec se control of viscosity is attained'by adjusting the conditionsin the hypochlorite bleaching treatment. It is understood that reductionof viscosity depends upon:

1. Temperature of the hypochlorite solution 2. Consistency of pulpslurry 3. Time of bleaching 4. Concentration of calcium hypochlorite 5.pH of the solution throughout the bleaching.

Of these, a variation of temperature, time and treated at 3% consistencyat C. for 3 hours in 7% sodium hydroxide solution and again Washed withwater. This 7% boiling treatment resulted in a pulp of considerablylighter color. The pulp was then bleached with 0.5% bleaching powder inthe form of calcium hypochlorite for 2 hour at about 20 C. The pulp wasthen dewatered, wash-ed and treated at 3% consistency for 1 hour at 20-25 C. in 13.7% sodium hydroxide solution. The pulp was then againthoroughly washed with water and treated with 13.7% sodium hydroxidesolution for 1 hour at 3% consistency at 20-25 C. All of the washingtreatments were carried out on an Allegheny 60 mesh wire screen, thusaccomplishing to a certain extent in each washing step a removal of apart of the undesirable fines."

The finished product had an analysisas follows:

Alpha per cent 99.4 Soda solubility do 2.7 Viscosity centipoises 15.1Ash per cent 0.23 Iron P. P. M. 43.9 Copper do 14.0 Bleachability percent 0.25 Pentosan do 2.5

This product is admirabiy suited for the preparation of high qualityviscose, cellulose nitrate, or even paper where great stability ofproduct is important.

Alpha cellulose is defined as that portion of a sample of cellulosicmaterial not dissolved by 17.5% sodium hydroxide solution at 20 0.,determined by a refinement of the method described by H. F. Lewis inTechnical Association Papers, series XVII, #1, 436 (\1934), described incopending application of Lyle Sheldon et al., S. N. 81,844, filed May26, 1936, pages 11-13.

Bleachability of cellulose, as used herein, is a measure of thematerials oxidizable by potassium permanganate in the presence of anacid under specific conditions, and is expressed in terms of standardbleaching powder containing 35% available chlorine. A detaileddescription of the method, including the conversion table for expressingthe permanganate number in terms of percent bleaching powder of 35%available chlorine, was published by T. A. P. P. 1., series XVII, #1,146 (1934), Permanganate Number of Pulp by R. N. Wiles.

Cuprammonium viscosity, as used herein, means the viscosity number orvalue obtained according to the following method: The cuprammoniumsolution was prepared by the action of air on electrolytic copper in thepresence of strong ammonia water. The copper concentration of thesolutions employed for viscosity determinations was 30, :2 g. per liter,and the ammonia content was 165 g., :2 g. per liter. The

concentration of cellulose employed was 0.6 g.

(oven dry basis) per 100 cc. of cuprammonium solution. The cellulosesample for this determination was dried at 70 C. to 4% moisture con.-tent. After weighing out 0.6 g. (oven dry basis), the sample wasmoistened, squeezed to a uniform weight of 2 g. and then dispersed incuprammonium solution in an atmosphere of hydrogen from which oxygen hadbeen completely removed. The viscosity measurements were made at 25 C.with a modified Ostwald pipet, constructed according to thespecifications of the American Chemical Society Committee on theViscosity of Cellulose (Journal of Industrial 8; Engineering Chemistry,I, #49; 1929). The time of flow in seconds was converted to centipoiseson the basis of the calibration of the pipet with oils of knownviscosity in centipoises obtained from the United States Bureau ofStandards.

This application is a continuation in part of application Serial Number39,113, filed September 4, 1935.

Having set forth the process in detail, what I which comprises graduallyremoving the noncellulosic content from wood by preparing an alkalinedigested unbleached pulp having the non-cellulosic content uniformlydistributed throughout the fibers by first boiling wood subdivisions inan alkaline delignifying liquor until the air is removed from the woodyinterstices and then heating saidisubdivisions submerged in saidalkaline liquor to form a completely defiberable pulp, chlorinating saidalkaline digested pulp with an amount of chlorine in the form ofchlorine water only sufllcient to react with the lignin content of thepulp, washing the pulp with water and thereafter gradually increasingthe alpha cellulose content and decreasing the soda soluble content ofthe fiber by subjecting the chlorinated pulp to a succession of chemicalrefining treatments in an alkaline medium including treat ment with analkaline solution of less than '7 percent concentration, treatment withhypochlorite solution at a pH maintained greater than 7.0 for a time andat a temperature sufilcient to control and lower the cellulose viscosityto the desired value, treatment with a caustic soda solution of 5percent to 9 percent concentration to remove n'on-cellulosie impurities.bleaching to whiteness with a dilute hypochlorite solution at a pHmaintained greater than 7.0, and treatment with a 19 percent to 18percent solution of caustic soda under mercerizing conditions, each ofsaid chemical refining treatments being followed by 1 a thorough waterwashing treatment.

2. The process as set forth in claim 1 in which the alkaline reagent inthe delignifying liquorand subsequent alkaline treatments is sodiumhydroxide.

FREDRICH OLSEN.

